The present invention relates to a light beam scanning device, and more particularly relates to a light beam scanning device by which reflection-scanning is conducted on a beam of light sent from a laser beam source, wherein a rotational polygonal mirror is used for the scanning operation.
Rotational polygonal mirrors are generally used for a light reflection means in an optical system of a light beam scanning device such as a laser beam printer and others. In the case where a rotational polygonal mirror is used in order to compensate the facet angle error, the following conditions are commonly maintained:
On a surface of the secondary scanning direction, a reflecting surface of the rotational polygonal mirror and a surface to be scanned are approximately set in a geometrical-optical conjugate relation, and a condensed light beam is incident upon a position close to the aforementioned reflecting surface. However, as the rotational polygonal mirror is rotated, the position of the reflecting surface is moved. As a result, the aforementioned conjugate relation can not be maintained. Therefore, an image formation point on the surface of the secondary scanning direction is shifted, so that an image surface curvature is caused with respect to the image formation point on the surface of the secondary scanning direction. Further, as illustrated in FIG. 1, a beam of light incident upon the aforementioned reflecting surface forms an angle .alpha. with respect to an optical axis of the scanning lens on a surface including the scanning lines. Accordingly, a rotational center of the rotational polygonal mirror is not located on the optical axis, so that the movement of the aforementioned reflecting surface is not symmetrical to the optical axis of the scanning lens. For this reason, in the case where a lens system symmetrical to the optical axis is used, it is difficult to appropriately correct the image surface curvature of the image formation point on the secondary scanning direction surface all over the scanning regions while consideration is given to the quantity of movement of the reflecting surface which is not symmetrical with respect to the optical axis.
When the size of a spot is .omega..sub.0 at a focal position in an optical laser beam system, and also when the size of a spot is .omega..sub.X at a position distant from a focal position by X, it is known that a relation between .omega..sub.X and .omega..sub.0 can be expressed by the following equation. ##EQU1##
In this case, .lambda. is the wave length of laser beams.
In order to obtain images of high quality all over the scanning regions, the fluctuation of the size of a spot of the secondary scanning direction is suppressed so that it can be maintained within a region of 10%. In this case, the results of calculation according to the equation (1) are described as follows.
In the case of EQU .omega..sub.0 =90 to 99 .mu.m, X=.+-.3.7 mm. (A)
In the case of EQU .omega..sub.0 =70 to 77 .mu.m, X=.+-.2.2 mm. (B)
In the case of EQU .omega..sub.0 =50 to 55 .mu.m, X=.+-.1.1 mm. (C)
As can be seen from the foregoing, when the resolution is improved, the allowable depth is reduced. In the case where the required resolution is low, practical problems are not caused even when a lens system symmetrical with respect to the optical axis is used. However, as the required resolution becomes high, the curvature of the image surface is increased in the case where the lens system symmetrical with respect to the optical axis is used.
A scanning device in which the image surface curvature is corrected all over the scanning regions is disclosed, for example, in Japanese Patent Publication Open to Public Inspection No. 49408/1991. In this scanning device, a deformed cylindrical lens shown in FIG. 4 is used, in which the radius of curvature rh in a section of the secondary scanning direction is determined as a function of the distance h from the optical axis. Therefore, this deformed cylindrical lens can provide a wide field angle which can not be provided by conventional cylindrical or toric lenses. However, in this case, a change in the radius of curvature is symmetrical with respect to the optical axis. Therefore, this scanning device can not correct a fluctuation of the image formation position on a surface of the secondary scanning direction which is generated in accordance with the fluctuation of a reflecting surface position when a rotational polygonal mirror is used as a reflecting system.
According to a scanning device disclosed in Japanese Patent Publication Open to Public Inspection No. 23313/1990, the radius of curvature on the secondary scanning direction surface is made to be non-symmetrical with respect to the optical axis in order to correct the fluctuation of the image formation position on a surface of the secondary scanning direction which is generated in accordance with the fluctuation of a reflecting surface position when a rotational polygonal mirror is used as a reflecting system. However, in this case, a lateral magnification of the scanning lens is not less than 3 and not more than 5. As described above, the magnification is high. Accordingly, it is required to manufacture the device highly accurately. As a result, the cost is raised. Further, since the lens including the non-symmetrical surface is made of glass, the manufacturing cost is raised from the view point of workability. Consequently, the above scanning device is not suitable for mass production so as to reduce the cost.